On Wed, 30 Nov 2016 13:44:18 -0800 (PST), "
wrote:

On Tuesday, November 29, 2016 at 9:37:28 PM UTC-5, Phil Allison wrote:

** I doubt they put the valves directly in liquid N2 - as it would destroy them instantly by cracking the glass. Even if done some other and safer way, the whole idea is completely nuts.

One of the design issues with valves is the vacuum tight seal between the pins and the glass base - cos steel and glass do not have the same tempco of expansion. Typically, steel has a higher tempco so the fit get tighter as the glass envelope heats - conversely it gets looser at very low temps.

Prolonged exposure at very low temps could let air ( or N2) into the vacuum and spoil the operation.

I do not believe they do, either. Here is one "Cryo" vendor describing their process:
_________________________________________________ ______________

The BLACK SABLE process is far more than putting tubes in a chamber and lowering the temperature. The BLACK SABLE process starts with tube selection. Only the best preamp, power and rectifier tubes are chosen to undergo this arduous process. All tubes undergo a 24 hour burn-in under load. The tubes are then placed in the Cryo-Processor where a computer-controlled proprietary system lowers the temperature to -300 Fahrenheit / -184 Celsius for 24 hours and then slowly ascends back to room temperature. Preamp tubes are tested and rated for Gain, Microphonics and Noise. Next, all 7/9 pin tubes have their pins cleaned for a better electrical connection. Power tubes are matched to within 3% for both Ip (Plate Current) and Gm (Transconductance). No other cryogenic process comes close. When only the best will do, choose BLACK SABLE.
_________________________________________________ _________________________

Which is 21F above the boiling point of liquid nitrogen, a cheap and readily obtainable material in liquid form. That being written, most metals, including coated steel, copper, cupro-nickel and other solderable materials have a vastly greater coefficient of expansion/contraction than even the softest glass (Pyrex). So either the pins are going to shrink out of their surroundings, or all this is done in a *hard* vacuum. There is no real danger of shattering the glass if done slowly, but as the pins will be *smaller* than the opening during the process - that is where all the 'magic' must take place.

Color me dubious.

Peter Wieck
Melrose Park, PA
Especially dubious because if the pins really did shrink away from the
glass the glass/metal seal would be broken and the tube would leak air
in once exposed to the atmosphere again. When tubes are made the glass
really does seal to the pins, just like glue. But glass is elastic
(more elastic than the metal pins) and the movement would be small so
maybe the glass does move with the pins. Steel pins on a tube would
shrink about .0001" going from room temp to 300 below. Glass would
move about .000067" so about 33 millionths of an inch difference. So
the glass/steel seal would probably hold.
Eric